This is a very strange bug. The SVG icons for the buttons are all being rendered white, and thus invisible against the white background. But when I inspect the element with Chrome’s DevTools, I can see no reason for this; their color is not set to white, and I can’t set any CSS overrides to make it anything but white. I’m totally stumped.
I just updated Discourse here, and moved it from the default “beta” track to the “stable releases” track so that updates will not have to happen as frequently. But it had no effect on this issue.
Also strange: I administer another Discourse forum that’s running the same version, and this bug is not present.
Perhaps it’s a plugin issue. I’ll look into that next…